Reduction of surface heating effects in nonlinear crystals for high power frequency conversion of laser light

ABSTRACT

A device for generating a frequency converted laser beam includes a nonlinear crystal having a first end face and a second end face opposed to the first end face. The nonlinear crystal is configured to receive at least one input laser beam at the first end face and output a frequency converted beam at the second face. A beam waist of the at least one input laser beam is positioned between the first end face and the second end face during a frequency conversion process. The device also includes a second crystal having a first end face bonded to the second end face of the nonlinear crystal and a second end face opposed to the first end face. A beam diameter of the frequency converted beam at the first end face of the second crystal is less than a beam diameter of the frequency converted beam at the second end face of the second crystal.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims benefit under 35 U.S.C. § 119(e) of U.S.Provisional Patent Application No. 60/794,521, filed Apr. 25, 2006,which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The demand for high power lasers in different wavelengths is growing asnew applications are found and the number of gain media in differentwavelength regions remains limited. The primary mechanism used to changelaser wavelengths to different spectral regions is the use of nonlinearcrystals to convert the fundamental laser wavelength to new wavelengths.Examples of such systems include the conversion of the wavelength at1064 nm from Nd:YAG lasers to a wavelength of 532 nm using nonlinearcrystals such as Potassium Dihydrogen Phosphate (KDP), Barium Borate(BBO), Lithium Triborate (LBO), Bismuth Borate (BiBO), and PotassiumTitanyl Phosphate (KTP). This light at 532 nm can be further convertedto 355 nm by summing the resultant 532 nm radiation with the remaininglaser fundamental at 1064 nm in another crystal to generate 355 nm. The532 nm can also be converted to 266 nm by doubling in crystals such asBBO, Cesium Dihydrogenarsenate (CDA), Potassium Fluoroboratoberyllate(KBBF) and Cesium Lithium Borate (CLBO). The 266 nm can be converted bysumming with the fundamental at 1064 nm to get to wavelengths as shortas 213 nm.

In all of these processes, it is theoretically possible to attainconversion efficiencies of the fundamental laser wavelength to thedesired wavelength range by as high as 100% for flat topspatial/temporal laser pulses. In practice, conversion efficiencies ashigh as 80-90% for second harmonic generation (SHG) and 30-40% for thirdharmonic generation (THG) to ultraviolet (UV) are attained usingspatial-temporal shaped pulses and/or effective multi-pass operation ofthe nonlinear crystals.

The above wavelengths have found an extremely wide array of applicationsfrom laser machining (e.g., marking/engraving/cutting) of materials, torange finding, to laser surgery/dental applications. The differentwavelengths have specific attributes related to the absorptivity of thelight in the medium of interest that reflect the importance in changingwavelengths. In some cases, the desired wavelength is for increasedtransparency, such as in the case of range finding. In other cases, itis for higher absorption, as in the case of highly confining the energyfor cutting materials through superheating and ablation. In all cases,the desired application benefits from higher laser power and frequencyconversion to higher powers in the wavelength of interest as higherpower either enables larger signals or faster processing.

Solid state lasers have been scaled to increasingly higher powers toattain significant brightness. For example, it is now possible now toproduce continuous (CW) lasers with diffraction limited output at 1 KW,which corresponds to a focusable average power of more than 10¹¹ W/cm².Despite the high power scaling capabilities of the fundamental lasersource, it has not been possible to scale the nonlinear frequencyconversion to take full advantage of the higher power inputs. Thus,there is a need in the art for methods and systems for reducing surfacedamage in nonlinear crystals used for efficient, high power frequencyconversion of laser light.

SUMMARY OF THE INVENTION

Embodiments of the present invention relate to methods and apparatusthat reduce surface heating effects in nonlinear crystals that are usedfor efficient high power frequency conversion of laser light.

A device for generating a frequency converted laser beam. The deviceincludes a nonlinear crystal having a first end face and a second endface opposed to the first end face. The nonlinear crystal is configuredto receive at least one input laser beam at the first end face andoutput a frequency converted beam at the second face. A beam waist ofthe at least one input laser beam is positioned between the first endface and the second end face during a frequency conversion process. Thedevice also includes a second crystal having a first end face bonded tothe second end face of the nonlinear crystal and a second end faceopposed to the first end face. A beam diameter of the frequencyconverted beam at the first end face of the second crystal is less thana beam diameter of the frequency converted beam at the second end faceof the second crystal.

According to another embodiment of the present invention, a device forgenerating a frequency converted laser beam is provided. The deviceincludes a cell having an entrance window and an exit window and anonlinear crystal having a first end face and a second end face opposingthe first end face. The nonlinear crystal is disposed in the cell withthe first end face a first predetermined distance from the entrancewindow and the second end face a second predetermined distance from theexit window. The nonlinear crystal is configured to receive at least oneincident laser beam incident on the first end face. Additionally, thenonlinear crystal is oriented to provide for phase matching for anonlinear optical conversion process operating on the at least oneincident laser beam and producing a frequency converted laser beam. Thedevice also includes a chemically inert fluid disposed in the cell. Thechemically inert liquid is substantially transparent to the at least oneincident laser beam and the frequency converted laser beam. A beamdiameter associated with the frequency converted laser light beam at theexit window is greater than a beam diameter associated with thefrequency converted laser light beam at the second end face.

According to yet another embodiment of the present invention, a devicefor generating a frequency converted laser beam is provided. The deviceincludes a nonlinear crystal having first crystal face and a secondcrystal face opposed to the first crystal face. The nonlinear crystal isconfigured to receive at least one incident laser beam at the firstcrystal face and perform a frequency conversion process on the at leastone incident laser beam to produce the frequency converted laser beampropagating in a propagation direction. The device also includes anoptical material having a first material face coupled to the secondcrystal face and a second material face opposed to the first materialface. The optical material is characterized by an index of refractionprofile varying in a direction transverse to the propagation direction.

According to an alternative embodiment of the present invention, adevice for generating a high power frequency converted laser beam isprovided. The device includes a nonlinear optical crystal having anentrance surface and a curved surface. The nonlinear optical crystal isconfigured to receive at least one laser beam at the entrance surfaceand provide a frequency converted beam through a frequency conversionprocess. The device also includes an optical coating coupled to thecurved surface. A reflectance of the optical coating is greater than 50%at a wavelength of the frequency converted beam. Additionally, a farfield divergence angle of the frequency converted beam increases afterreflection off the curved surface. The device further includes a heatsink in thermal contact with the optical coating.

According to another alternative embodiment of the present invention, adevice for generating a frequency converted laser beam is provided. Thedevice includes a nonlinear optical crystal having an entrance surfaceconfigured to receive at least one laser beam. The nonlinear opticalcrystal supports a frequency conversion process used to generate afrequency converted laser beam. The device also includes one or moresecond optical materials coupled to the nonlinear optical crystal andconfigured to provide a propagation path for the frequency convertedlaser beam. The at least the nonlinear optical crystal or the one ormore second optical materials is characterized by a curved surface suchthat a far field divergence angle of the frequency converted laser beamincreases after reflection from the curved surface. The device furtherincludes a reflective coating optically coupled to the curved surface. Areflectance band of the reflective coating is associated with thefrequency converted laser beam. Moreover, the device includes a heatsink in thermal contact with the curved surface.

Numerous benefits are provided by embodiments of the present invention.For example, some embodiments provide devices and methods that reduce oreliminate surface heating at one or more surfaces of a frequencyconversion crystal and thus allow for stable, high power frequencyconversion processes. Additionally, embodiments provide for beamexpansion during propagation in order to reduce fluence levels atinterfaces and corresponding crystal damage. In particular embodiments,Brewster surfaces and integrated diverging elements are utilized toprovide for long-lived operation and low insertion losses. Dependingupon the embodiment, one or more of these benefits may exist. These andother benefits have been described throughout the present specificationand more particularly below. Various additional objects, features andadvantages of the present invention can be more fully appreciated withreference to the detailed description and accompanying drawings thatfollow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an LBO crystal optically bonded to a sapphire crystal withhigh thermal conductivity according to an embodiment of the presentinvention;

FIG. 2 shows a phase matching LBO crystal optically bonded to a secondnon-phase matching LBO crystal according to an embodiment of the presentinvention;

FIG. 3 shows an LBO crystal in an inert transparent liquid for heatremoval according to an embodiment of the present invention;

FIG. 4 shows an LBO crystal bonded to a gradient index component whoseindex profile is designed to increase the beam diameter by forming adiverging lens according to an embodiment of the present invention; and

FIG. 5 illustrates an LBO crystal bonded to a Brewster angle cut thermalconductor (e.g. sapphire crystal or non-phase matching LBO) in whichinsertion loss on the exit face is eliminated.

FIG. 6 shows an LBO crystal with one of its ends shaped as a sphericallyconvex inward surface according to an embodiment of the presentinvention;

FIG. 7 illustrates an LBO crystal with a shaped surface according toanother embodiment of the present invention; and

FIG. 8 illustrates an LBO crystal with a shaped surface according to yetanother embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

One issue that has prevented scaling of the nonlinear frequencyconversion to higher powers is related to damage induced in thenonlinear crystal as the power is increased. As a result, the nonlinearfrequency conversion efficiency is limited by damage to the nonlinearcrystals. The fundamental limit is defined by the peak power density ofthe input laser beam that can cause damage by dielectric breakdown.Materials have been engineered that have both sufficiently highnonlinear coefficients and bulk peak power damage thresholds to attainhigh conversion and high power. Despite these gains, the maximumachievable output power into harmonics or other frequency conversionprocesses are still limited by surface damage. The intrinsic damagethresholds at surfaces are greatly reduced relative to the bulk due topoorer thermal conductivity at the surface. As a result, even smallresidual absorption leads to stress fracture at the unconfined surfacelayer. Thus, the ultimate power output for any given nonlinear crystalis determined, in part, by surface damage.

The damage arises from small residual absorption that leads to crystalheating, deformation, creation of defects, and further absorption thatleads to optical damage and obscuring of the crystal's transparency andconversion ability. The damage is particularly a problem for crystalswith AR coatings that are routinely used to prevent the Fresnel loss onthe air/crystal interface. Optical coatings are in general prone tooptical damage due to mismatch of the materials, defects, impurities,and the like. The surface of even uncoated crystals act as the weakestpoint in the overall power conversion process as the damage threshold ofthe surface is intrinsically smaller than the bulk crystal.

The most well known example of damage to nonlinear crystals is thedamage of crystals used in high power UV generation for Q-switched lasersystems with pulse durations on the order of 10-100 ns. There has been acurrent market identified for high power UV for cutting circuit boards,direct writing/repairing semiconductor wafers, drilling and marking onplastics or metals. However, it has been found that above 3 kW/cm², or3-5 Watts of average power for typical focusing conditions, the crystalsdamage. The damage occurs below the peak power that would normally leadto crystal damage and the damage is confined primarily to the exitsurface.

A few methods have been attempted/implemented in high power laserindustry to solve the damage problem. The most common solution has beento use a crystal with a large cross section and translate the crystal ina pre-defined pattern to expose a fresh area when one area is damaged.The crystal is replaced after there is no longer enough useable area.Although this technique increases the useful lifetime of a singlecrystal, it does not address the fundamental aspects of the crystaldamage and involves a bulky, complicated and costly mechanical motionmechanism.

Another approach is to cut the exit face of the THG crystal at theBrewster angle. The AR coating on the crystal surface is thus eliminatedwith the proper configuration of the beam polarization and the damagethreshold of the surface is increased. The Brewster surface also servesas spatial beam separator, making it possible to alleviate theconventional dichroic mirrors for separation of UV beam from theincident beams. This method is useful in some regards but does notaddress the inherent damage mechanism at that surface.

The inventors have discovered the mechanisms of crystal damage. Weidentified at least four possible explanations for crystal damage. Thesepossibilities include: 1) lower damage thresholds at the surface; 2)crystal strain at the surface due to outgassing from the crystal latticeand/or polishing effects; 3) increased rate of oxidation reactions atthe surface due to heating and/or UV absorption by oxygen and impuritiesin the air; and 4) heating/transport problems unique to the surface.

By enclosing the crystal in a vacuum chamber in which oxygen could beremoved and replaced with inert noble gases, it was discovered that thesurface damage was not due to reactions. The introduction of oxygen didnot significantly affect the laser induced surface damage rate from thatobserved in air. However, when the gas was completely removed so thatthe only thermal transport and cooling of the surface was throughthermal diffusion through the crystal, the damage to the surface wasimmediate. By reintroducing various gases that provided even moderatecooling of the surface, the rate of laser damage was retarded. Byflowing nitrogen (N₂) gas over the surface the rate of damage wasfurther reduced and up to 3 Watt of UV was useable at typical focusingconditions giving greater than 3 kW/cm² of UV power at the exit surface.

Without being limited by any particular theory, the above studiesillustrated that the damage mechanism is due to small impurities at thesurface that lead to heating of the surface. Due to the fact that thesurface has very poor thermal conductivity and can only lose heatprimarily through collision exchange with gas molecules at the interfaceor thermal conduction through the crystal that has poor thermalconductivity, the surface temperature rises. This surface temperatureincrease causes crystal strain and darkening, further absorption, andrun-away heating to damage. A solution utilized by embodiments of thepresent invention is to provide a means to reduce or avoid heatdeposition at the interface to enable the highest possible scaling inaverage power for any given class of peak power laser and wavelengthconversion range. Without limiting embodiments of the present invention,we believe it is the rate of heating per unit area that makes the mostsignificant impact on surface heating. Thus, embodiments of the presentinvention expand the area of the frequency converted beam to spread theheat over a larger area and thereby minimize the rise in surfacetemperature before it encounters a surface.

According to an embodiment, a high thermal conductor/beam transportmaterial is optically bonded to or placed in optical contact with theexit face of a nonlinear crystal to increase the output power at theconverted wavelength for a range of conversion frequencies.

In the case of composite optical materials, two or more opticalcomponents can be bonded together. Such composite materials have beenused for various optical functions. Examples are birefringent crystalbased polarizers, beamsplitters, and zero-th order waveplates. There areseveral methods of bonding of two or more optical components (referredto herein as “optical bonding”). The most common method of opticalbonding is epoxy bonding, in which the ends of the optical componentsare coated with a thin layer of epoxy, brought into contact, and thencured by heat or UV light. However, epoxy bonding cannot withstand highpower laser radiation due to the intrinsically low damage threshold andlow transmission at UV of the epoxy itself.

Epoxy-free (non-adhesive) components are generally desirable for highpower optical applications. The basic method for non-adhesive opticalbonding is “optical contact,” in which two optical components areoptically smoothed on at least one surface of each of the components andthe two optically smooth surfaces are brought into close contact at roomtemperature. Under these extremely smooth conditions, the van der Waalsand other inter-atomic and molecular forces are maximized and a bond isthus formed by atomic and molecular attractions between the surfaces.Variations of optical contact have also been developed to createextremely robust and mechanically strong bonds. Of special interest toembodiments of the present invention are optical contacting withchemical activation and diffusion bonding. In the chemically activatedoptical bonding (see. for example, C. Myatt, N. Traggis, and K. L.Dessau, Laser Focus World, Vol. 42, 95(2006)), two optically polishedcomponents are treated chemically on the surfaces to create danglingbonds and then brought into optical contact. The part is then annealedat a temperature well below the melting temperatures to form covalentbonds between the atoms of each surface. The chemically activatedbonding method can be used to form a robust, transparent, and large-areabond not only between similar but also dissimilar optical materials. Thebonding can even be formed between coated surfaces.

In diffusion bonding, as disclosed in U.S. Pat. No. 5,846,638, twocomponents are first optically contacted and then heat treated at anelevated temperature. This heating step allows the atoms at bothsurfaces to move to their most stable configuration in the interfaceregion which upon cooling results in a much stronger bond than byoptical contact alone. Diffusion bonding also has the ability to form arobust, transparent bond not only between similar but also dissimilaroptical materials. Diffusion bonded laser rods have been used as gainmaterials for very high power lasers (see, for example, S. A. Payne etal, “Diode Arrays, Crystals, and Thermal Management for Solid-StateLasers”, IEEE J. of Selected Topics in Quantum Electronics, Vol. 3,71(1997)).

Referring to FIG. 1, an embodiment of the present invention includes adevice to produce a high power frequency converted laser beam thatincludes a sapphire crystal 10 (which has a thermal conductivity aboutone order of magnitude larger than those of most nonlinear opticalcrystals) that is optically bonded to an LBO crystal 12 for high powerUV generation with the crystals being jointed at an interface 16. TheLBO crystal 12 has an orientation that provides phase matching for thenonlinear optical conversion process, whereby at least one input laserbeam 21 generates a frequency converted output laser beam 14.

The frequency conversion process is a function of the light intensity inthe nonlinear optical crystal. Intensity is often defined as the ratioof the optical power to the beam area, which is related to the square ofthe beam diameter. Therefore, in some embodiments of the presentinvention, the one or more laser beams 21 are focused in the nonlinearoptical crystal such that the focal point, often denominated as the beamwaist, is located between end face 19 and interface 16. The dimensionsof the beam waist depends on the application at hand and the opticalpower available. For example, for input laser beam peak power of lessthan 25 kW, the beam waist can be in the range of less than 1 μm to 5mm; for input laser beam peak power much higher than 25 kW, the beamwaist can reach 10 cm. The diameter of the frequency converted beam canbe calculated using known equations. To a first approximation, the beamdiameter is similar to the diameter of the input laser beam. Crystal 12has end face 19 and the face at interface 16 which are optically flat,coated or uncoated, as are the end faces for crystal 10, showing endface 14 through which the high power frequency converted laser lightexits the crystal.

The optical bonding is such that a uniform consistent bond is obtainedover the entire radiated area to avoid voids that locally reduce thethermal conductivity and lead to thermal damage, which can occur at asolid/gas interface. In some embodiments, the interface 16 ischaracterized by an optical flatness of less than λ/2 and a scratch-digcharacteristic better than 80/60. A solid/solid contact has much higherthermal conduction than a solid/gas interface, thereby eliminating thetemperature rise at the interface and thus gives the same damagethreshold as the bulk. The sapphire crystal itself is also subject tothe same damage mechanism at its surface. The sapphire crystal 10 bondedto the LBO crystal 12 is long enough to transport the laser light beamfar enough such that the resulting expanded beam produces little to nodamage at the exit surface 14 of the optically bonded sapphire crystal10. The exit face 14 may be uncoated or coated to reduce the Fresnelloss on the face. The nonlinear crystal 12 and the sapphire crystal 10each have a longitudinal axis 17, which are coextensive and the incidentlaser beam 21 is directed into the end face 19 of the nonlinear crystal12 normal to the plane of the end face 19 and the high powered laserbeam produced in crystal 12 travels parallel to the longitudinal axis 17of the two crystals to emerge from exit face 14.

It is noted that while the device shown in FIG. 1 is configured so thatthe (one or more) incident laser beam(s) are normal to the opticallyflat end face 17 of crystal 12, it will be understood by those skilledin the art that other configurations are included within the scope ofthe present invention. For example, the beam may be deviated slightly inorder to achieve the optimum angular phase matching or/and to reduce thedirect back-reflection. Thus, variations in angle deviation from normalincidence are included within some embodiments of the present invention.In some embodiments, the angle of deviation is less than 60° for eitherthe input laser beam or the frequency converted laser beam. One ofordinary skill in the art would recognize many variations,modifications, and alternatives.

The preferred length of the sapphire crystal 10 depends on the laserbeam parameters and surface damage threshold of the sapphire crystal 10at the exit surface 14 at the particular frequency converted wavelengthof light of the laser beam. Fresnel loss occurs on the interface 16between two materials with difference in refractive index and can beexpressed as well known in the art as [(n1−n2)/(n1+n2)]² where n1 and n2are the indices of refraction of the materials. At a wavelength of 1064nm, the Fresnel loss at the LBO/sapphire interface is about 0.3%,equivalent to the loss introduced by anti-reflection (AR) coated optics(e.g. lenses, waveplates, Q-Switch modules, and the like) and may beignored or managed for most laser applications.

Embodiments of the present invention provide a novel scheme for couplingoptical elements that are transparent at the wavelengths of interest andlong enough to avoid excessive surface heating by transporting the beamto a surface where the laser beam area is large enough to avoid surfacedamage. It is desirable in applications involving high power UVgeneration to transport the laser beam far enough into the coolingmaterial to allow the beam to expand. All materials absorb to a certaindegree in the UV and experience the same problem as the nonlinearcrystal used to generate the UV laser beam in the first place. It shouldbe noted that the crystals, such as LBO, which are selected for thisapplication are some of the most UV transparent materials known. Thistransparency is useful as even minute absorption leads to bulk heatingand deleterious degradation of the laser beam as well as nonlinear powerdependent losses and instabilities.

The much greater sensitivity of the surface to minute absorption andtemperature increases exacerbates the problem. In addition, surfacedefects are generally impossible to avoid and this fundamentally leadsto increased absorption of UV in the surface region. The phenomenon ofUV generated surface damage has been found to be a universal problem.Even highly transparent materials such as LBO, fused silica, sapphire,calcium fluoride (CaF₂), which would normally be considered to be immuneto absorption induced damage, are observed to experience surface damageat high power UV levels. This observation is a new phenomenon that hasonly been observed with the recent advance of high brightness UV laserbeams. It is not sufficient to put on a sapphire end cap to cool thesurface of the crystal used to generate the UV or to expect the UVdamage threshold of the sapphire crystal to be usefully higher. Whatevermaterial is chosen will experience the same mechanism of surface damage.Thus, simple diffusion bonded end caps described in S. A. Payne et al,“Diode Arrays, Crystals, and Thermal Management for Solid-State Lasers”,IEEE J. of Selected Topics in Quantum Electronics, Vol. 3, 71(1997) forsurface cooling simply transfer the surface damage to the exit surfaceof the transparent capping layer.

While the embodiment illustrated in FIG. 1 has been described using anLBO crystal, it will be understood this is for purposes of illustrationonly and the nonlinear crystal may be any material that provides thedesired optical conversion process. Examples of nonlinear crystalsinclude, but are not limited to, Potassium Dihydrogen Phosphate (KDP),Barium Borate (BBO), Lithium Triborate (LBO), Bismuth Borate (BiBO),Potassium Titanyl Phosphate (KTP), Cesium Dihydrogenarsenate (CDA),Potassium Fluoroboratoberyllate (KBBF), Cesium Lithium Borate (CLBO),and Potassium Titanyl Arsenate (KTA).

The embodiment illustrated in FIG. 1 uses optical bonding to effectivelytransport the beam in a transparent body until the beam divergencecreates a beam that is sufficiently large to avoid the optical damage bythe high power frequency converted laser beam at the exit surface of thefinal medium. In some embodiments, the frequency converted beam has adivergence in the range between 0.1 mrad to 250 mrad. The beam expansioncan be calculated by those skilled in the art given the input laser beamparameters and indices and lengths of the crystals. In some embodiments,the beam is expanded by a factor of greater than one. In a particularembodiment, the beam is expanded by a factor of the square root of two.This particular reference is given as the thermal transport at a surfacediscontinuity is one half that of the any given point in the bulk of thematerial assuming the surface is exposed to a vacuum. Decreasing thebeam area by a factor of two compensates for this intrinsic difference.Other embodiments provide for a range of beam expansion from one (1) toone hundred (100).

In relation to embodiments of the present invention, beam propertiesincluding beam radius, beam width, beam waist, beam waist position, beamdiameter, and beam divergence are utilized. Generally, definitions asprovided according to the ISO are utilized herein. For example, the beamdiameter may be defined as the distance between 1/e² intensity points.Because this is not typically applicable to multimode beams includingflat-top beams, other definitions are utilized as appropriate to theparticular application. Thus, in an embodiment in which the beamdiameter increases as a beam propagates from a first plane to a secondplane, various definitions as appropriate to the particular beam spatialdistribution may be utilized in a consistent manner.

Additionally, the concept of a far field divergence angle is utilized todescribe beams utilized and produced in embodiments of the presentinvention. According to some embodiments, the far field divergence angleis defined with reference to a certain point on the beam propagationaxis as the divergence angle that the beam would have if propagated fromthat point through free space to a point that is 100 km away.

Additionally, although sapphire is used as crystal 10 in the deviceillustrated in FIG. 1, it will be understood by those skilled in the artthat the second crystal could be any other crystal exhibiting highthermal conductivity and which is substantially transparent at thewavelengths of interest. According to embodiments of the presentinvention, the term substantially transparent is used to denotematerials that are characterized by low levels of optical absorption ata wavelength of interest. For example, in some embodiments, an opticalintensity is reduced less than about 20% on passing through asubstantially transparent material. The crystal 10 may therefore be anyone of, but not limited to, sapphire, Yttrium Aluminum Garnet (YAG),Gadolinium Gallium Garnet (GGG), Gadolinium Vanadate (GdVO₄), CalciumFluoride (CaF₂) and Cubic Zirconia crystals.

In other embodiments, to reduce or eliminate surface heating, phasematched crystals are optically bonded to non-phase matched crystals forconcurrent beam transport and cooling. To completely or substantiallyeliminate contact problems associated with the method of optical contactor variations on this method, thermal conductor may be of the samematerial as the frequency conversion materials, but not contribute tothe frequency conversion process. Using diffusion bonding as an example,it involves mechanically holding the two sections of crystal in place atan elevated temperature until the lattice atoms at the contact regionrearrange and bond to each other thereby forming a bond between the twooptical elements. In this process, lattice matching issues that aresatisfied in order to avoid strain and defects that may lower the damagethreshold and thus deteriorate the optical performance of the frequencyconversion device.

A solution to avoid lattice mismatch problems is to make both sectionsof the coupled optical device from the same crystal, such as illustratedin FIG. 2 where both sections 20 and 22 are made from LBO crystals. LBOcrystal section 20 has an orientation that allows optimal phase matchedfrequency conversion in this crystal. LBO crystal section 22 should havean orientation deviated from phase matching to prevent further UV orother nonlinear conversion. In the embodiment illustrated in FIG. 2, thesecond LBO section 22 does not contribute to further frequencyconversion as the primary crystal 20 will be chosen for an optimallength for highest frequency conversion. Lengths of the primary crystal20 longer than the optimal frequency conversion length may lead to beamellipticity, back conversion, and loss in laser brightness. The purposeof the second LBO section 22 is to transport the beam far enough so thatthe exit surface 24 is not exposed to the light beam until it hasexpanded to a large enough area to be below the damage threshold when itexits surface 24. Since the phase matching is highly sensitive to theorientation change of the crystal (e.g. the angular acceptance for THGof 1064 nm in LBO is less than 0.2 deg-cm) while other physicalproperties such as the effective refractive indices, the effectivethermal expansion, and the effective lattice constants are much moretolerant of the same change, the orientation deviation in the LBOcrystal section 22 may be made sufficiently small to suppress phasematching while at the same time retaining the near-perfect match incrystal lattices, thermal expansion, and refractive indices to theprimary LBO crystal section 20. This leads to diminished mechanicalstress and strong bonding on the interface 26. The Fresnel loss on theinterface 26 is virtually nonexistent due to disappearance of the indexstep at the interface.

The two nonlinear crystals 20 and 22 each have a longitudinal axis 17which is coextensive and the incident laser beam 21 is directed into theend face 23 of the nonlinear crystal 20 normal to the plane of the endface 23 and the high powered laser beam produced in crystal 20 travelsparallel to the longitudinal axis 17 of the two crystals to emerge fromexit face 24. However, as discussed with reference to the embodimentshown in FIG. 1 the incident laser beam(s) do not need to be incidentexactly at normal incidence.

A novel feature provided by the embodiment illustrated in FIG. 2 is thatthe use of identical materials with a slight mismatch in crystalorientation substantially eliminates any residual problems associatedwith strain and index difference at the interface between the nonlinearcrystals while providing near-perfect match of all the latticeproperties.

For many crystals, it is not possible or practical to use other crystalsto remove surface heat and transport the beam due to non-availability ofthe materials, high cost, or poor optical bonding qualities. The use ofinert liquids that are transparent at the wavelengths of the input laserbeam and the frequency converted beam can be used instead to accomplishsimilar effects. For example, referring to FIG. 3, in another embodimentof the present invention, one can use liquid chlorofluorocarbons (CFCs)or other liquids or gases which exhibit high transmittance in the UVdown to about 240 nm. In some embodiments, the fluid is characterized bya density greater than 0.1 g/cm³. In addition to being transparent andhaving reasonably high heat capacities, CFCs are also inert liquids,i.e., they do not react with the LBO crystal and exhibit virtually nophotochemistry at the transparent wavelengths. The index of refractionof the CFC may be chosen to be index matched to that of the nonlinearcrystal 30 in order to reduce the Fresnel loss at the interface betweennonlinear crystal exit face 31 and the liquid. However, it is notnecessary that they be perfectly matched as the major benefit is coolingof the nonlinear crystal exit face by having a high density fluid,compared to a solid/gas interface, to enable heat transport away fromthe nonlinear crystal exit face 31.

As shown in FIG. 3, the LBO crystal 30, with the orientation optimizedfor efficient phase matched frequency conversion, is supported in aliquid circulation cell 32 having an inlet 34 and exit 36 through whichthe CFC liquid or other liquid or gas is circulated into and out of cell32. Windows 40 and 42, which are transparent to the incoming andoutgoing beams of light, respectively, are located at opposed ends ofcell 32. An additional element, diverging lens 44, may be insertedbetween the crystal 30 and window 42 to ensure a rapid expansion of thelight beam to avoid damage to window 42.

The nonlinear crystal 30 has a longitudinal axis 35, which is normal tothe first and second opposed end faces 31 and 33, and the nonlinearcrystal 30 is mounted in the cell 32 such that the incident laserbeam(s) 21 are directed into the end face 33 of the nonlinear crystal30. The high power UV laser light beam travels parallel to thelongitudinal axis 35 of the nonlinear crystal 30. However, as discussedwith respect to the embodiment of the device shown in FIG. 1 theincident laser beam(s) do not need to be incident exactly at normalincidence. The embodiment illustrated in FIG. 3 has the distinctadvantage of unlimited scaling in the length of the beam transportsection located between crystal exit face 31 and the cell exit window 42with low cost material and is universally applicable to all nonlinearcrystals with the correct choice of transparent inert liquids at theconversion wavelength of interest.

In some applications, they may be problems with the amount of materialneeded to transport the light beam far enough from the crystal face toget the intensity of the laser beam below the damage threshold of thecrystals at the exit surface for the highest power operation for anygiven laser system. As the power is scaled higher, the beam diameter ofthe laser will be expanded and the confocal parameter of the laser beamwill be correspondingly increased. This increase means that the requireddistance of propagation in the light transporting material (e.g., theoptically bonded highly conducting solid 10 in FIG. 1, the opticallybonded non-phase matched material 22 in FIG. 2, or the inert transparentliquid in FIG. 3) will increase quadratically with the beam diameter.

FIG. 4 shows an LBO crystal bonded to a gradient index component whoseindex profile is designed to increase the beam diameter by forming adiverging lens according to an embodiment of the present invention. Toavoid the need for excessive amounts of material, an index of refractionprofile can be introduced into the transport medium 50 attached to forexample the LBO crystal 12 as shown in FIG. 4. Thus, embodiments utilizea gradient index beam expander that is optically bonded to the frequencyconversion crystal. The average index of refraction of the opticalmaterial measured over the graded index profile is substantially equal(e.g., within 10% or within 5%) of the index of refraction of thenonlinear crystal in an embodiment of the present invention.

The index of refraction profile of the transport material 50 may be inthe plane transverse to the axis of the transport medium 50 and designedbased on the laser input parameters to rapidly increase the beamdiameter by forming a diverging lens. This diverging lens can beachieved, for example, by using a transport material wherein the indexof refraction profile increases as a function of a radial dimension. Inthe case of solid state contacts, this index of refraction profile canbe accomplished by ion exchange, chemical-vapor-deposition (CVD),thermal diffusional glass preforming with gradient index layers, laserinduced index changes, or the like. In some embodiments, gradient indexbeam expanders fabricated using techniques used for fabricating lightwaveguides in solids are employed. One of ordinary skill in the artwould recognize many variations, modifications, and alternatives.

The transport medium 50 is an optical material which may be a crystalwith the index profile written into it or it may be an amorphousglass-like material which provides for ease of fabrication. The averageindex of refraction of the profile may be designed to be close to thatof LBO by proper selection of the substrate material in order tominimize the Fresnel power loss on the interface 52. In an embodimentthat uses a fluid to provide cooling and beam transport, a diverginglens or a beam expander, coupled to the crystal by the inert transparentfluid can be introduced in the liquid cell. In this case, the index ofrefraction of the fluid is designed not to index match the beamexpanding optics in order to enable the beam expansion while matchingthe crystal with some compromise in Fresnel losses at the variousinterfaces.

The nonlinear crystal 12 and the beam transport medium 50 each have alongitudinal axis 17 which are coextensive and the incident laserbeam(s) 21 are directed into the end face 51 of the nonlinear crystal 12normal to the end face 51. The high powered laser beam produced incrystal 12 travels parallel to the longitudinal axis 17 of the twomaterials to emerge from exit face 53 as an expanded laser beam.However, as discussed with respect to the embodiment shown in FIG. 1 theincident laser beam(s) do not need to be incident exactly at normalincidence.

In all of the above embodiments, the incident laser beams are normal ornear normal to the output surfaces of thermal conductors. Referring toFIG. 3, the surfaces 14, 24 of thermal conductors or the window 42 ofthe liquid cell 32 may be uncoated or coated depending on the requiredpower and the laser system design (e.g. intracavity or extracavityfrequency conversions). In general, coatings (e.g., anti-reflection (AR)coatings) are used to reduce the Fresnel reflection loss at the surface(˜4% for LBO and ˜8% for sapphire at the wavelength of 355 nm). Suchcoatings provide for effective extraction of the generated photons andto prevent back-reflection, which may harm optical components in thelaser system. While the reduction or elimination of surface heating andthe larger beam spot on the output faces compared to a conventionalfrequency converter increases the damage threshold on the surface, theAR coated surfaces are generally more susceptible to optical damage thanuncoated surfaces due to material mismatch, defects, or impurities inthe coating layers. Thus, some embodiments of the present inventionutilize a design with a surface that does not use AR coating and at thesame time does not introduce excessive loss.

U.S. Pat. No. 5,850,407 describes a third harmonic generator with anuncoated Brewster exit face directly made on the frequency conversioncrystal. As is well known in the art, the laser beam passes through aface at Brewster angle essentially without loss in power with a properarrangement of the polarization (i.e., p-polarization), thus eliminatingthe need for an AR coating. Due to the dispersive nature of the Brewstersurface, it also provides a means for wavelength separation without useof a coated dichroic mirror.

In an embodiment, the transport medium 50 is also a nonlinear crystalhaving a graded index profile. In this embodiment, the transport medium50 is oriented at an angle deviated from a phase matching condition tothereby to reduce a magnitude of a frequency conversion process in thetransport medium. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives.

In another embodiment of the present invention, the exit surface onthermal conductor is cut at the Brewster angle. FIG. 5 shows a designwith the Type II phase matched THG generation in the nonlinear crystalLBO 60 as an example. By optically bonding a Brewster-cut thermalconductor 62 to the nonlinear crystal 60, advantages are providedincluding taking advantage of both enhancement of the damage thresholdoffered by the reduction or elimination of surface heating at theinterface surface 64 and robustness of an uncoated surface 66. Theobliqueness of Brewster surface 66 increases the effective beam area onthe exit surface 66 by a factor of 1/sin(arctan n) (where n is the indexof refraction of thermal conductor) due to deflection of light on thesolid/air interface. With a sapphire crystal 62 as the conductor, forexample, the expanding factor is close to 2. Transport of the light beamin thermally conducting material 62 also expands the beam size asdiscussed in the previous embodiments. The reduction of laser power perunit area on the exit surface 66 further reduces the possibility ofdamage on the uncoated exit surface 66. This unique combination offavorable features in surface heating reduction, increased beam spotsize, and diminished insertion loss enables a frequency converter with ahigh damage threshold and reliability generally unachievable by othertechniques.

The nonlinear crystal 60 has a longitudinal axis which is normal to theend face 68. The nonlinear crystal 60 is mounted such that the incidentlaser beam is directed into the first opposed end face of the nonlinearcrystal 68 normal to end face 68, but as with the previous embodiments,it will be understood that the incident laser beam(s) do not need to beincident exactly at normal incidence.

The dispersive Brewster angle surface 66 on thermal conductor 62 can beused for spatial separation of the converted beam from the incidentlight beams. A distinct advantage of the Brewster angle on thermalconductor 62 rather than the nonlinear crystal 60 is that the spatialseparation is now the sole property of thermal conductor and a largeseparation can be achieved with the selection of a thermal conductorthat has a large dispersion, thus facilitating the design of the wholefrequency conversion system.

As an example, the separation by a sapphire thermal conductor is 2°compared to 1° by an LBO crystal in the case of THG of 1064 nm. Also,this design may be used for intracavity frequency conversions as well asextracavity conversions. As is well known in the art the conversionefficiency in the case of intracavity application is highly sensitive tothe insertion loss to the fundamental power of the nonlinear component.The excessive insertion loss results in high laser oscillating thresholdand drastic reduction in the converted laser power. Utilizing thefrequency converter designs described herein, there is no loss on theBrewster angle output surface 66. Accordingly, the input surface 68 maybe AR coated as is done with conventional optics, since the fundamentalwavelength incident on the crystal does not damage the surface. Thenonlinear crystal/thermal conductor interface 64 may introduce a losswhich can be expressed as [(n1−n2)/(n1+n2)]² where n1, and n2 are theindices of refraction for the nonlinear crystal and thermal conductor,respectively. This loss can be substantially eliminated when the thermalconductor has the same index as the frequency conversion crystal. As anexample, the thermal conductor may be the same material as the frequencyconversion crystal, but with a small deviation in orientation (asdiscussed above with the embodiment in FIG. 2). With a sapphire crystalbonded to LBO, the loss on interface 64 is ˜0.3% at 1064 nm, which isequivalent to the loss by a typical AR coated element in the cavity andmay be ignored or managed for most laser applications.

It is noted here that the use of a Brewster angle may introduceastigmatism. Beam reshaping and compensation may be used to removeastigmatism in the output light beam if so required. In particular, amatched Brewster cut thermal conductor placed in a sufficient distancemay be used to compensate the astigmatism arising from the first thermalconductor.

As discussed throughout the present specification, accelerated beamexpansion may be used to avoid the need for excessive amounts ofmaterial for a sufficient beam size at the exit face. In order for theUV beam to expand sufficiently while still propagating through thecrystal, an additional optical element with negative optical power canbe incorporated within the crystal. One way to do this, as describedwith reference to FIG. 4 is to create spatial profile of index ofrefraction in a way that the profile acts as a negative lens. Beamexpansion can also be accelerated by forming at least one curved opticalsurface in the crystal with a high reflection coating for the UV lightapplied at that surface. The UV beam is positioned in the crystal to hitand reflect from the crystal curved surface at least once and thecurvature of the curved surface is designed so that reflected UV beamgets expanded before it hits the exit surface. The reflection surfacemay contain its own defects that will be heated, but that surface can becooled externally by putting it in contact with a material with highthermal conductivity (i.e., a heat sink) that can quickly spread theheat generated by these defects and hence decrease the problem withdamage at that point.

Referring to FIG. 6, in an embodiment, the curved surface isincorporated into the nonlinear crystal itself. Here, incident laserbeam(s) 21 enters a nonlinear crystal 70 through a crystal opticalsurface 73 and a portion of their energy gets converted into anultraviolet (UV) beam 29 through nonlinear optical conversion inside thenonlinear crystal. The nonlinear crystal has a convex (looking from thebulk of the crystal) spherical optical surface 71 that is formed in theside of the nonlinear crystal that stands against the propagation of theUV beam. As a result, the convex face reflects the UV beam into thecrystal and accelerates beam expansion before the beam exits from thecrystal.

Shaping spherical surfaces in optical materials is well known to thoseskilled in the art. The surface 71 is coated with a high reflectioncoating for the UV beam wavelength and preferably for the wavelengthscontained in the laser beam 21 as well. For example, the coatingreflectivity is greater than 50%. In other embodiments, the coatingreflectivity is preferably greater than 99%. The radius and the positionof the center of curvature of the spherical surface 71 is designed suchthat the UV beam gets reflected backwards at a small angle that allowsangular separation of the incoming and outgoing beams outside thenonlinear crystal.

According to some embodiments, the design also provides that no UVconversion in the reflected beams is performed as the conversion processwill be optimized for the incoming beam(s). Since the curved opticalsurface 71 is convex, the UV beam will get expanded during thepropagation through the nonlinear crystal 70 before it hits the crystalsurface 73 and leaves the crystal. Therefore, the power density at theUV beam exit surface will be reduced and the UV damage power thresholdwill be increased for the exit surface. On the other hand, the surface71 will be hit by high intensity UV beam and heated, but that surfacecan be easily cooled by an external heat sink 72. The heat sink can beany material with good thermal conductivity and heat capacity that isput in good thermal contact. In some embodiments, the thermal contactbetween the heat sink and the surface to be cooled is referred to asbeing in thermal communication. Thus, physical contact between the heatsink and the surface may or may not be provided depending on theparticular application. Thermally conductive layers may be interposedbetween the heat sink and the surface in other embodiments. One examplefor the heat sink is a metal coating on the top of the high reflectioncoating. Typical metal coatings have thermal conductivity two orders ofmagnitude larger than typical nonlinear crystals, so can quickly spreadthe heat across their surface. For very large UV powers, an additionalheat sink fabricated from metal or other materials can be pressed orsoldered to the metal coating for a more efficient heat removal.

The amount of the UV beam expansion depends on the propagation distance,so it is advantageous to increase the propagation distance by bondingthe nonlinear crystal to a thermal conductor and incorporate a curvedsurface on thermal conductor. The additional optical material providesan additional path length for the UV beam expansion. Also, the thermalconductor is not a nonlinear optical crystal and the material may bechosen for ease of curved surface fabrication.

An embodiment of the present invention incorporating a curved surface ona thermal conductor is illustrated in FIG. 7. The incident laser beam(s)21 enter a nonlinear crystal 80 through a nonlinear crystal opticalsurface 85. UV beam 29 is generated by nonlinear optical conversioninside the nonlinear crystal. The side of the nonlinear crystal that isopposite to the surface 85 is optically bonded to a second opticalmaterial 82 with good optical and thermal properties through aninterface 81. Thus, the embodiment illustrated in FIG. 7 employs some ofthe concepts illustrated in FIG. 6, but with the LBO crystal opticallybonded to a thermal conductor along with a spherical surfaceincorporated directly in the thermal conductor.

The choice of the second optical material and ways of bonding to thenonlinear crystal should follow the same guidelines as the ones outlinedthroughout the present specification. The second optical material has aspherical surface 83 drilled in it, which is coated with a highreflection coating for the UV laser beam and is put in good thermalcontact with a heat sink 84. The relative position of the sphericalsurface 83 with respect to the UV beam 29 is such that the UV beam getsreflected backwards and leaves the nonlinear crystal through the surface85 with a small angle with respect to the incoming beams 21.Accordingly, an angular separation of the incoming and outcoming beamsis provided in some embodiments. Generally, the design should alsoensure no UV conversion occurs in the reflected beams as the conversionprocess will be optimized for the incoming beam(s).

As discussed before, minute absorption exists even in the mosttransparent nonlinear crystal and thus may heat up the nonlinearcrystal. Therefore, it is advantageous to redirect the generated intenseUV beam away from the nonlinear crystal (where the frequency conversiontakes place) into thermal conductor and exit from thermal conductor. Anembodiment that illustrates this idea is presented in FIG. 8. Laserbeam(s) 21 enter a nonlinear crystal 91 and generate an UV beam 29inside the nonlinear crystal through parametric conversion. Thenonlinear crystal is optically bonded through an interface 92 to athermal conductor 94 that has a spherical surface 95 drilled into it.The spherical surface is coated with high reflection coating for the UVbeam and it is brought into close contact with a heat sink 93. Asillustrated in FIG. 8, the beam reflected from the curved facepropagates in thermal conductor.

Discussions in the previous embodiments related to designs of nonlinearcrystal 80, thermal conductor 82, their interface 81, spherical surface83, the high reflection coating on the surface 83, and the heat sink 84also apply to corresponding elements 91, 94, 92, 95, and 93. In FIG. 8,the position of the spherical surface 95 with respect to the UV beam 29is chosen so the UV beam gets reflected under a large angle of incidencerelative to the normal of the curved surface. In an embodiment, theangle of incidence is approximately 45°. In other embodiments, the angleof incidence varies over a range of angles, for example, from about 1°to about 89°. In this way, the UV beam is directed away from the regionwith large frequency conversion and the problems of extra heating of thenonlinear crystal by residual UV absorption are reduced or eliminated.The length of the material 94 in the direction of propagation of the UVbeam reflected of the surface 95 is long enough to allow sufficientexpansion of the UV beam before it exits at surface 97 of the material95.

It is noted that in embodiments illustrated in FIGS. 6-8, it may beadvantageous to choose a reflection plane (the plane containing both theincident and reflected beams) at the spherical surfaces to beperpendicular to the polarization of the UV beam. In this way, thepolarization of the UV beam will not be effected through the subsequentpropagation through the nonlinear crystals or the attached opticalmaterial. This is useful in case specific polarization of the UV beamcoming out the nonlinear crystal or the optical material is required. Itis also noted that the use of a curved surface may introduceastigmatism. Beam reshaping and compensation may be used to removeastigmatism in the output light beam if so required. Also, it may beadvantageous for the embodiments illustrated in FIGS. 6-8 to include anoptical material that is brought in close optical contact with thenonlinear crystal or the attached optical material and having theBrewster cut surface that acts as the exit surface for the frequencyconverted beam. The advantages include further increases in the beamarea and increasing angular separation between the useful frequencyconverted beam from other beams.

The above embodiments of the invention are relevant to all nonlinearconversion processes from infrared (IR) to visible and from the visibleto UV spectral ranges, i.e. the issues are fundamental to all conversionprocesses and all laser classes. The excessive heating experienced atthe beam exit surface as opposed to the bulk of the nonlinear crystalmedium acts to limit the maximum power extractable from a nonlinearoptical system. Thus, embodiments of the present invention enable thepower extracted to reach the fundamental limit as defined by the damagethreshold of the bulk of the crystal and not the surface. The overalleffect can be gains of more than a factor of two in output power withlittle to no damage and, for many cases, more than an order of magnitudeincrease in achievable output powers.

While the present invention has been described and illustrated withrespect to generating a high power UV laser beam using nonlinear opticalconversion processes to convert a longer wavelength laser beam incidenton a nonlinear crystal into the high power UV laser beam, those skilledin the art will appreciate that the present method also applies to allnonlinear frequency conversion in which there is surface heating andrelated damage. Merely by way of example, embodiments of the presentinvention are also applicable to high power infrared (IR) generationthrough optical parametric amplification (OPA) or difference frequencygeneration (DFG), in which surface heating will limit output powers innext generation intense IR laser systems.

Thus the high power laser light beam that is produced from the incidentlaser beam or beams entering the nonlinear crystal through opticalparametric amplification (OPA) or difference frequency generation (DFG)(the produced beam) has a wavelength in the IR region of the lightspectrum. Both OPA and DFG are two nonlinear optical processes whichproduce longer wavelengths in the infrared (IR) from typically two inputbeams with shorter wavelengths. The multiple input beams may also havedifferent wavelengths.

While the present invention has been described with respect toparticular embodiments and specific examples thereof, it should beunderstood that other embodiments may fall within the spirit and scopeof the invention. The scope of the invention should, therefore, bedetermined with reference to the appended claims along with their fullscope of equivalents.

1. A device for generating a frequency converted laser beam, the devicecomprising: a nonlinear crystal having a first end face and a second endface opposed to the first end face, wherein the nonlinear crystal isconfigured to receive at least one input laser beam at the first endface and output a frequency converted beam at the second face, wherein abeam waist of the at least one input laser beam is positioned betweenthe first end face and the second end face during a frequency conversionprocess; and a second crystal having a first end face bonded to thesecond end face of the nonlinear crystal and a second end face opposedto the first end face, wherein a beam diameter of the frequencyconverted beam at the first end face of the second crystal is less thana beam diameter of the frequency converted beam at the second end faceof the second crystal, wherein the nonlinear crystal and the secondcrystal comprise a same nonlinear crystal material and the secondcrystal has an orientation deviated from a phase matching condition tothereby to reduce a magnitude of the frequency conversion process in thesecond crystal.
 2. The device of claim 1 wherein the second end face ofthe nonlinear crystal and the first end face of the second crystal arecharacterized by an optical flatness of better than λ/2 at 632 nm and ascratch-dig characteristic better than 80/60.
 3. The device of claim 1wherein the nonlinear crystal and the second crystal comprise a bondinterface substantially free of an adhesive.
 4. The device of claim 3wherein the bond interface comprises at least one of a chemicallyactivated interface or a diffusion bonded interface.
 5. The device ofclaim 1 wherein the second end of the second crystal is coated with atleast one of a single-layer or a multiple-layer dielectric coating. 6.The device of claim 1 wherein the nonlinear crystal is selected from thegroup consisting of Potassium Dihydrogen Phosphate (KDP), Barium Borate(BBO), Lithium Triborate (LBO), Bismuth Borate (BiBO), Potassium TitanylPhosphate (KTP), Cesium Dihydrogenarsenate (CDA), PotassiumFluoroboratoberyllate (KBBF), Cesium Lithium Borate (CLBO), andPotassium Titanyl Arsenate (KTA).
 7. The device of claim 1 wherein awavelength associated with the at least one input laser beam ischaracterized by a first wavelength and a wavelength associated with thefrequency converted beam is characterized by a second wavelength lessthan the first wavelength.
 8. The device of claim 7 wherein the secondwavelength is a UV wavelength.
 9. The device of claim 1 wherein thefrequency conversion process comprises at least one of an opticalparametric amplification (OPA) process or a difference frequencygeneration (DFG) process, the at least one input laser beam beingcharacterized by a first wavelength and the frequency converted beambeing characterized by an IR wavelength greater than or equal to thefirst wavelength.
 10. The device of claim 1 wherein the second end faceof the second crystal is cut at a Brewster's angle for a wavelength ofat least the at least one input laser beam or the frequency convertedbeam.
 11. The device of claim 1 wherein a magnitude of the frequencyconversion process in the second crystal is substantially zero.
 12. Thedevice of claim 7 wherein the first wavelength and the second wavelengthin the non-linear crystal remain substantially the same as in the secondcrystal.
 13. A device for generating a frequency converted laser beam,the device comprising: a nonlinear crystal having a first end face and asecond end face opposed to the first end face, wherein the nonlinearcrystal is configured to receive at least one input laser beam at thefirst end face and output a frequency converted beam at the second face,wherein a beam waist of the at least one input laser beam is positionedbetween the first end face and the second end face during a frequencyconversion process; and a second crystal having a first end face bondedto the second end face of the nonlinear crystal and a second end faceopposed to the first end face, wherein a beam diameter of the frequencyconverted beam at the first end face of the second crystal is less thana beam diameter of the frequency converted beam at the second end faceof the second crystal, wherein the second end face of the second crystalis cut at a Brewster's angle for a wavelength of at least the at leastone input laser beam or the frequency converted beam.
 14. The device ofclaim 13 wherein the second end face of the nonlinear crystal and thefirst end face of the second crystal are characterized by an opticalflatness of better than λ/2 at 632 nm and a scratch-dig characteristicbetter than 80/60.
 15. The device of claim 13 wherein the nonlinearcrystal and the second crystal comprise a bond interface substantiallyfree of an adhesive.
 16. The device of claim 15 wherein the bondinterface comprises at least one of a chemically activated interface ora diffusion bonded interface.
 17. The device of claim 13 wherein thesecond crystal is selected from the group consisting of Sapphire,Yttrium Aluminum Garnet (YAG), Gadolinium Gallium Garnet (GGG),Gadolinium Vanadate (GdVO₄), Calcium Fluoride (CaF₂) and Cubic Zirconiacrystals.
 18. The device of claim 13 wherein the second end of thesecond crystal is coated with at least one of a single-layer or amultiple-layer dielectric coating.
 19. The device of claim 13 whereinthe nonlinear crystal is selected from the group consisting of PotassiumDihydrogen Phosphate (KDP), Barium Borate (BBO), Lithium Triborate(LBO), Bismuth Borate (BiBO), Potassium Titanyl Phosphate (KTP), CesiumDihydrogenarsenate (CDA), Potassium Fluoroboratoberyllate (KBBF), CesiumLithium Borate (CLBO), and Potassium Titanyl Arsenate (KTA).
 20. Thedevice of claim 13 wherein a wavelength associated with the at least oneinput laser beam is characterized by a first wavelength and a wavelengthassociated with the frequency converted beam is characterized by asecond wavelength less than the first wavelength.
 21. The device ofclaim 13 wherein the frequency conversion process comprises at least oneof an optical parametric amplification (OPA) process or a differencefrequency generation (DFG) process, the at least one input laser beambeing characterized by a first wavelength and the frequency convertedbeam being characterized by an IR wavelength greater than or equal tothe first wavelength.
 22. The device of claim 13 wherein a latticeconstant of the second crystal is substantially equal to a latticeconstant of the nonlinear crystal.
 23. The device of claim 13 wherein anindex of refraction of the second crystal is substantially equal to anindex of refraction of the nonlinear crystal.
 24. A device forgenerating a frequency converted laser beam, the device comprising: anonlinear crystal having a first end face and a second end face opposedto the first end face, wherein the nonlinear crystal is configured toreceive at least one input laser beam at the first end face and output afrequency converted beam at the second face, wherein a beam waist of theat least one input laser beam is positioned between the first end faceand the second end face during a frequency conversion process; and asecond crystal having a first end face bonded to the second end face ofthe nonlinear crystal and a second end face opposed to the first endface, wherein a beam diameter of the frequency converted beam at thefirst end face of the second crystal is less than a beam diameter of thefrequency converted beam at the second end face of the second crystal,wherein the second crystal is selected from the group consisting ofSapphire, Yttrium Aluminum Garnet (YAG), Gadolinium Gallium Garnet(GGG), Gadolinium Vanadate (GdVO₄), Calcium Fluoride (CaF₂) and CubicZirconia crystals.
 25. The device of claim 24 wherein the second endface of the nonlinear crystal and the first end face of the secondcrystal are characterized by an optical flatness of better than λ/2 at632 nm and a scratch-dig characteristic better than 80/60.
 26. Thedevice of claim 24 wherein the nonlinear crystal and the second crystalcomprise a bond interface substantially free of an adhesive.
 27. Thedevice of claim 26 wherein the bond interface comprises at least one ofa chemically activated interface or a diffusion bonded interface. 28.The device of claim 24 wherein the second end of the second crystal iscoated with at least one of a single-layer or a multiple-layerdielectric coating.
 29. The device of claim 24 wherein the nonlinearcrystal is selected from the group consisting of Potassium DihydrogenPhosphate (KDP), Barium Borate (BBO), Lithium Triborate (LBO), BismuthBorate (BiBO), Potassium Titanyl Phosphate (KTP), CesiumDihydrogenarsenate (CDA), Potassium Fluoroboratoberyllate (KBBF), CesiumLithium Borate (CLBO), and Potassium Titanyl Arsenate (KTA).
 30. Thedevice of claim 24 wherein a wavelength associated with the at least oneinput laser beam is characterized by a first wavelength and a wavelengthassociated with the frequency converted beam is characterized by asecond wavelength less than the first wavelength.
 31. The device ofclaim 30 wherein the second wavelength is a UV wavelength.
 32. Thedevice of claim 24 wherein the frequency conversion process comprises atleast one of an optical parametric amplification (OPA) process or adifference frequency generation (DFG) process, the at least one inputlaser beam being characterized by a first wavelength and the frequencyconverted beam being characterized by an IR wavelength greater than orequal to the first wavelength.
 33. The device of claim 24 wherein alattice constant of the second crystal is substantially equal to alattice constant of the nonlinear crystal.
 34. The device of claim 24wherein an index of refraction of the second crystal is substantiallyequal to an index of refraction of the nonlinear crystal.
 35. The deviceof claim 24 wherein the second end face of the second crystal is cut ata Brewster's angle for a wavelength of at least the at least one inputlaser beam or the frequency converted beam.